Hot filling is common for certain special contents, in particular those undergoing pasteurization (fruit juice or other fruit beverages, milk, condiments) or an infusion (tea, coffee).
The temperature of this type of contents commonly exceeds 90° C., while the containers in question are ordinarily manufactured from PET (polyethylene terephthalate), whose glass transition temperature is on the order of 80° C.
Hot filling poses a problem of mechanical strength of the container because of its softening due to its exposure to the heat of its contents.
A known technique making it possible to increase the mechanical strength of the container is heat-setting, which consists, at the end of the forming of the container, in keeping it temporarily at a high temperature (for example, by keeping it in contact with the heated wall of its mold) in such a way as to increase the crystallinity of the material by thermal means.
This technique certainly makes it possible to increase the mechanical performance levels of the container during the filling by imparting to it good mechanical strength, but it may nevertheless prove inadequate. Actually, even when heat-set, the container, once stoppered, cannot stand up to the stresses induced by the contraction of the contents accompanying its cooling. The result is often uncontrolled deformations that degrade both the aesthetics of the container and its mechanical strength (in particular when it has to be palletized).
To monitor the deformations induced by the stresses accompanying the contraction of the cooling contents, a first approach consists in equipping the container with deformable panels, cf., for example, the U.S. Pat. No. 5,704,503 (Continental PET). The panels deform under the pressure prevailing in the container. They swell during filling, and then become hollow when the contents cool. The main drawback of this approach is the limit that the presence of the panels imposes on the freedom of shape of the container, to the detriment of its appearance.
A second approach consists in equipping the container with a membrane bottom that can be put back, cf., for example, the U.S. Pat. No. 8,671,653 (Graham Packaging), in which the membrane is put back by means of a mechanical pusher, in such a way as to reduce the internal volume of the container to put the contents back under pressure and thus to acquire rigidity. This second approach is better than the first in terms of structural rigidity of the container and the freedom of shape for the former, but it imposes manufacturing stresses.
Actually, it happens that the membrane is unstable in its returned position and regains its initial position under the pressure prevailing in the container. A valid hypothesis is that the container has not cooled sufficiently, such that the membrane that is put back cannot reach the desired position that ensures its stable locking in position.
Allowing all of the containers to cool freely for a long time would hamper productivity (remember that the usual throughput today is on the order of 50,000 containers per hour).
An alternative approach would be to subject all of the containers to forced cooling (for example, by vaporization), but this approach consumes energy.